Spontaneous emission is the process of photon emission by a quantum system as it transitions from an excited state to a ground state. The excited state lifetime is determined by the spatial overlap between the excited and ground state wavefunctions, and photonic density of states that is seen by the emitter. In quantum systems used as sources of spontaneous emission—such as molecules, quantum dots and semiconductor quantum wells—this lifetime is typically on the scale of 1-10 ns, corresponding to rates of 100-1,000 MHz. This relatively slow rate of spontaneous emission is limited both by the small physical size of the emitters and the low photonic density of states of free space. For photonic devices that are based on light emission, these long radiative lifetimes are a hindrance to high-speed devices.
A spontaneous emission source of particular interest for device applications is semiconductor quantum dots (QDs). These emitters combine a tunable emission wavelength at room temperature, high radiative quantum efficiency, excellent photostability, and ease of integration with other materials. For example, colloidal QDs have been demonstrated as stable, room-temperature single-photon sources, but the slow radiative rate associated with these systems limits the attainable repetition rate. Likewise, light emitting diodes are not used in telecommunications, in part due to the long spontaneous emission lifetimes. QDs are also promising as gain media for micro- and nanoscale lasers, but achieving a low lasing threshold has proven challenging due to non-radiative Auger recombination outcompeting the slow intrinsic radiative lifetime of ˜20 ns.
To increase the rate of spontaneous emission of QDs, a range of approaches have been developed to engineer the photonic environment of the emitter and increase the photonic density of states. The figure of merit that characterizes the enhancement in the spontaneous emission rate is the Purcell factor, FP=γsp/γsp0, where γsp0 is the intrinsic spontaneous emission rate and γsp is the enhanced rate. Dielectric cavities coupled to epitaxial QDs were first used for Purcell enhancement and improved emission directionality. However, obtaining large Purcell factors, FP (˜Q/V), in dielectric cavities demands high quality (Q) factors and small mode volumes (V). Earlier work has shown that significant fabrication effort is required to position a single QD at the maximum field of the cavity and to spectrally tune the QD emission to match the cavity mode. Despite advanced techniques to fabricate and tune high-Q cavities, including micropillar cavities, microtoroid resonators, and photonic crystal cavities, experimental values of the Purcell factor in dielectric optical cavities are presently limited to ˜75. Additionally, these typically narrow band systems often require low temperatures and are not well suited for tailoring the broadband emission from room temperature emitters. However, room-temperature modulation of an LED based on a photonic crystal cavity has shown modulation rates of up to 10 GHz.
Plasmonic nanocavities, such as bowties, dimers, and film-coupled nanoparticles, have attracted interest in recent years because they offer large field enhancements, broad resonances (typical Q factors ˜10-30), room-temperature operation and, in some cases, can be easily fabricated via colloidal synthesis. Plasmonic nanocavities support strong field enhancements and a strongly modified photonic density-of-states, thus providing a flexible means of controlling the spontaneous emission rate of quantum emitters and other light-matter interactions at the nanoscale. Typical drawbacks of plasmonics include losses due to non-radiative decay in the metals and limited control over the directionality of emission. Various plasmonic structures have been utilized to enhance the emission of QDs, but so far only limited Purcell factors of less than 145 have been demonstrated. Higher Purcell factors of up to 1,000 have been obtained for molecules, but such large enhancements of QDs have so far proven elusive. Furthermore, in plasmonic structures the Purcell enhancements are typically accompanied by low radiative efficiency due to significant non-radiative losses, or have low directionality of emission. For example, hybrid QD and Au nanoparticle structures assembled by atomic force microscopy nanomanipulation have shown Purcell factors up to 145 but radiative decay rate enhancements of only ˜8. One-dimensional metamaterials with a hyperbolic dispersion have also been used to achieve control of spontaneous emission, but the Purcell factors have been limited to ˜10.